The development of high-brilliance X-ray sources coupled with advances in manufacturing technologies have led to significant improvements in sub-micrometre probes for spectroscopy, diffraction and imaging applications. The generation of a small beam spot size is commonly based on three principles i) total reflection (as used in optical elements involving mirrors or capillaries), ii) refraction (such as in refractive lenses) and iii) diffraction. The latter effect is employed in Bragg­Fresnel and Soret lenses or Fresnel zone plate lenses (FZP). These FZP lenses currently give the best spatial resolution, but their applications are traditionally limited to rather soft X-rays. In fact, the efficiencies of such FZPs are still far from unity because a significant fraction of incident beam is delivered into the (undiffracted) zero-order. At multi-keV energies, the efficiencies are usually of the order of 10-20% and cause a significant loss in the potential photon flux in the spot. This drawback can be overcome by optimising the structure profile of the zones. As theoretically shown by Tatchyn in 1970 [1], a 100% focusing efficiency can be obtained by a non-absorbing lens with a parabolic zone profile. In addition to their high efficiency, these lenses offer the advantages of low background signal and effective reduction of unwanted diffraction orders by introducing selection rules [2].

In practice, such profiles are extremely difficult to produce with existing lithography techniques. In this work, a multi-step profile zone (Figure 124) approximated to the parabolic profile. A quaternary electroplated Nickel zone-plate ­ developed in collaboration with CNR-IESS (Rome) and TASC-INFM (Trieste) ­ was tested on the X-ray microscopy beamline that can operate over an energy range of 2 ­ 8 keV (Figure 125). The FZP efficiency measurements were performed at 5.5, 6.0, 7.0 and 8.0 keV and the efficiencies were 43%, 45%, 57% and 45%, respectively. The measured optical gain (the ratio between the photon density delivered by the FZP at the focus and the photon density impinging on the FZP) for the nickel FZP is about 2,500 at 7 keV. If we were to work at the diffraction limit by reducing the focal length, a flux gain higher than 8000 could be anticipated. These lenses should have a significant impact on techniques such as microscopy, microfluorescence and microdiffraction which require medium spatial resolution (500 ­ 100 nm) and high flux at fixed energies.

[1] T. Tatchyn, R. O. in X-ray Microscopy, 43, eds G. Schmahl, D. Rudolph, Springer Series in Optical Sciences, Springer, Berlin, 40­50 (1990).
[2] W. Yun, B. Lai, A.A. Krasnoperova, E. di Fabrizio, Z. Cai, F. Cerrina, Z. Chen, M. Gentili, E. Gluskin, Rev. Sci. Instrum. 70(9), 3537-3541 (1999).

Principal Publication and Authors
E. Di Fabrizio (b), F. Romanato (b), S. Cabrini (b), M. Gentili (c), B. Kaulich (a), J. Susini (a), R.Barrett (a), Nature 401, 895 (1999).

(a) ESRF
(b) ELETTRA, Trieste (Italy)
(c) IESS/CNR, Rome (Italy)